1. Field of the Invention
This invention relates to hydraulic cylinder jacks, and in particular to hydraulic staging jacks.
2. Discussion of the Prior Art
Hydraulic staging jacks are well known. Such jacks are used for lifting, separating or any application of hydraulic force where the hydraulic force is applied by alternately jacking and cribbing. For example, when jacking up a building in preparation for moving the building, the hydraulic cylinder starts out at a relatively low position relative to a base with the base supported on supports (cribbing) and the cylinder pushing against I-beams on which the building is resting. The building is first jacked up a limited distance by applying hydraulic pressure to the cylinder and the building is blocked at that position. Cylinder pressure is then relieved and the operator moves the cylinder up in its base and fixes it at a new axial position relative to the base. Hydraulic pressure is then reapplied to the cylinder to once again extend the cylinder and move the building up higher. This cycle is repeated until the building reaches its desired height.
Prior art hydraulic staging jacks typically required the operator to operate two levers for engaging and disengaging the cylinder with the base. This was inconvenient and relatively difficult to do, and also permitted improper usage, since operators could lock the cylinder with only one lever.